The cortical collecting duct (CCD) plays an important role in systemic acid-base homeostasis. Recent studies show the majority of rabbit superficial CCD intercalated cells possess both apical and basolateral plasma membrane C1-/HCO3- exchange. Since this cell has both apical and basolateral C1-?HCO3-, it might be able to either secrete or reabsorb HCO3- and therefore function as either an A cell or B cell. Yet no studies have directly examined the role this cell plays to regulate CCD HCO3- transport. The central hypothesis of this application is that intercalated cell with both apical and basolateral C1-/HCO3- exchange has the capability to transport HCO3-, but that it is not active under normal conditions. When stimulated this cell may be able to either secrete or reabsorb HCO3-. This cell may play a major role in systemic acid-base physiology. Two major aims will examine this hypothesis. the first aim is to characterize the major H+ and HCO3- transporters of this cell. the presence and membrane location of Na+/H+ exchange, H+-ATPase, and H+-K+- ATPase will be determined because of the critical role they play in intracellular pH (pHi) regulation and in HCO3- transport. Apical and basolateral plasma membrane C1-/HCO3- exchange will be compared to C1- /HCO3- exchange in the A cell and B cell because this cell may represent an intermediate intercalated cell, able to function as either an A cell or B cell. The second aim will be to examine the role this cell plays to regulate CCD HCO3- transport. Studies will examine whether this cell can secrete HCO3- and whether it also can reabsorb HCO3-. Finally, we will examine whether in vivo metabolic acidosis and alkalosis induce conversion of this cell into either an A cell or B cell. These studies will measure intracellular pH in individual intercalated cells of in vitro perfused CCD to examine the location and activity of specific HCO3- and H+ transporters. Some experiments will combine this with electron microscopic examination, including immunohistochemical localization of H+-ATPase and C1-/HCO3- exchange, of the same CCD. These techniques will expand our understanding of the mechanisms by which the CCD regulates systemic acid-base homeostasis.